1. Field of the Invention
The present invention relates to the preparation of an oxidation catalyst, and, more especially, to the preparation of an oxidation catalyst employed in the conversion of hydrogen sulfide into sulfur.
The invention also relates to the catalyst, per se, thus obtained.
2. Description of the Prior Art
It is known to this art to convert hydrogen sulfide into sulfur by a conventional process comprising the following two reaction stages: EQU H.sub.2 S+3/2 O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O EQU SO.sub.2 +2H.sub.2 S.revreaction.3/.sub.x S.sub.x +2H.sub.2 O
The first stage consists of oxidation of hydrogen sulfide into sulfur dioxide.
The second stage is a catalytic conversion stage which is conventionally carried out by means of a Claus catalyst well known to those skilled in this art, on contact with which the SO.sub.2 reacts with H.sub.2 S to form sulfur.
In conventional installations for the production of sulfur from "acid gas", namely, from gas containing hydrogen sulfide originating either from natural gas or from refinery gas, the first stage is a heating stage. The acid gas and a stoichiometric amount of oxygen are introduced into a reactor in a manner such that an effluent composed of one-third of SO.sub.2 and of two-thirds of H.sub.2 S is obtained at the outlet of the reactor. An amount of sulfur vapor is formed at the same time, and this must be eliminated before the gas passes to the second stage, namely, to the Claus catalysis stage.
The heating stage of oxidation of H.sub.2 S in the presence of air or oxygen is carried out at a minimum temperature of 900.degree. C. This temperature is easy to maintain, since the reaction is very exothermic provided that the amount of hydrogen sulfide in the gas to be treated attains a value of 15%, preferably at least 35%, by volume.
For values below this limit, it is not possible to maintain a flame temperature sufficient to obtain stable combustion without resorting to significant process modifications.
Furthermore, attempts have been made for many years to solve the problem of purifying acid gases containing less than 35% by volume of H.sub.2 S without having to consume enormous amounts of energy in order to maintain the flame temperature at not less than 900.degree. C.
It too is known, from U.S. Pat. No. 4,092,404, that the aforesaid oxidation stage can be carried out using an oxidation catalyst based on vanadium deposited onto a non-alkaline porous refractory support. This oxidation reaction must be carried out by operating strictly at a temperature below 450.degree. C. (850.degree. F.) and is applicable to gases in which the content of H.sub.2 S is less than 3%.
Now, the oxidation of H.sub.2 S to provide SO.sub.2 is highly exothermic and thus requires a complex system for controlling the temperature and severe technological constraints. The catalyst is rapidly deactivated at temperatures above 450.degree. C.
On the other hand, it is known from published European Patent Application No. 39,266 that industrial effluents containing sulfur can be oxidized to sulfur dioxide by catalytic oxidation in the presence of a metal, for example, iron, combined with a porous support. A temperature higher than that mentioned in the above patent cannot be used with this catalyst. Contrariwise, the process described in this patent relates exclusively to the oxidation of hydrogen sulfide to sulfur dioxide and in no way relates to the simultaneous reaction between hydrogen sulfide and sulfur dioxide in order to produce sulfur, per se.
Gases with a low content of H.sub.2 S are easily treated by catalytic oxidation, because they do not entail a significant rise in the temperature of the catalyst; however, if the content of H.sub.2 S exceeds 3% by volume, an increase in the temperature of the catalyst and deactivation of the latter occur.
According to the prior art, it is thus possible to treat gases containing more than 35% by volume of H.sub.2 S in a stage which is solely thermal, or gases containing less than 3% of H.sub.2 S in a stage which is solely catalytic, but the prior art does not solve the problem consisting of treating gases containing between 3 and 35% of H.sub.2 S.